The US President has described his second state visit to Britain as "one of the highest honours of my life".
Donald Trump was speaking at a banquet in Windsor Castle hosted by King Charles.
"It's a singular privilege to be the first American president welcomed here.
"This is truly one of the highest honours of my life, such respect for you and such respect for your country.
"For many decades, His Majesty the King has epitomised the fortitude, nobility and the spirit of the British monarchy and the British people."
President Trump described the UK and US as "two notes in one chord ... each beautiful on its own, but really meant to be played together".
King Charles praised the US leader's "personal commitment" to seeking peace.
"Our countries are working together in support of crucial diplomatic efforts, not least of which, Mr President, is your own personal commitment to finding solutions to some of the world's most intractable conflicts, in order to secure peace.
"Mr President, Mrs Trump - the bond between our two nations is indeed a remarkable one.
"Forged in the fire of conflict, it has been fortified through our shared endeavours and burnished by the deep affection between our people.
"Tested time and again, it has borne the weight of our common purpose and raised our ambition for a better world.
"So in renewing our bond tonight, we do so with unshakeable trust in our friendship and in our shared commitment to independence and liberty," the monarch said.
Earlier, President Trump and his wife Melania, joined King Charles, Queen Camilla and other royals and dignitaries at Windsor for a carriage procession, with the route lined by 1,300 British service personnel.
The president, occasionally chatting and smiling with the king, inspected a guard of honour.
Later, the couple viewed historical items from the British Royal Collection relating to the US, and visited St George's Chapel, the final resting place of Queen Elizabeth, where he laid a wreath on her tomb.
The queen, who hosted Mr Trump for his first state visit in 2019, died in September 2022.
There was a further military parade and a flypast by Britain's Red Arrows aerobatics team, but poor weather meant that UK and US military jets - a symbol of bilateral defence collaboration - were unable to join.
Mr and Mrs Trump also held a private meeting with Prince William - described by the president as "very handsome" and his wife Kate. The prince's spokesperson said the encounter was "warm and friendly".
A small group of supporters gathered outside Windsor Castle, west of London, to welcome President Trump, including one man wearing a cap that read: "Trump was right about everything".
Former New York police officer Steven DeFranco said he knew that he had to make a stop there during his business trip when he heard that the US leader was on a state visit.
The 64-year-old said the US leader is "doing a fabulous job and described him as a "beaming light".
Meanwhile, in the centre of the city, thousands of people protested against the visit.
The 'Trump Not Welcome' demonstration was organised by the Stop Trump Coalition and supported by other organisations, including Amnesty International and Abortion Rights, as well pro-Palestinian activists.
"I quite simply dislike everything that Trump and his administration represent around the globe. (They are) absolutely awful," said Bryan Murray, a retiree, who attended with his wife and held a placard reading 'Dump Trump'.
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More than 1,600 police officers were deployed to deal with the protest, which moved peacefully towards parliament in Westminster and featured banners reading: "Not Wanted Here, Not Wanted Anywhere" and "Trump, a big step back on the evolution of man".
Police said that around 5,000 people took part.
A spokesperson for the Stop Trump Coalition said the rally was a chance to show the world that "Britain rejects hate, division and authoritarianism".
The turnout was similar to that seen for Mr Trump's previous state visit in 2019, but much lower than the figure for his first official visit to Britain as president in July 2018, when estimates varied between tens of thousands and 250,000.
Four people were arrested yesterday after images were projected onto Windsor Castle of President Trump alongside convicted sex offender Jeffrey Epstein.
The issue could surface during the visit after British Prime Minister Keir Starmer sacked Peter Mandelson as UK ambassador to the United States over Epstein ties last week.
President Trump and his wife Melania are being hosted by King Charles at Windsor, the oldest and largest inhabited castle in the world and a home to British monarchs for almost 1,000 years.
Officials laid on what they said was the largest military ceremonial welcome for a state visit in living memory, and Mr Trump, an overt royal fan, has made little secret of his delight at being not just the first US leader, but the first elected politician to be invited for two state visits.
He said that he loved Britain, describing it as "a very special place".
Mr Starmer is hoping to use that sentiment to Britain's advantage as his government seeks to cement the two nations' "special relationship", deepen economic ties, secure billions of pounds of investment, discuss tariffs and press President Trump on Gaza and Ukraine.
The UK and US agreed a £31 billion tech pact to mark the visit.
Mr Starmer also wants further progress on trade, after the UK secured the first deal with Mr Trump to lower some tariffs.
Talks between the two men tomorrow may include remaining levies on steel, whisky and salmon.
The UK wants "to see if they can refine the trade deal a little bit," President Trump said yesterday.
Mr Starmer will host the US leader at his country residence in Chequers, around 65km north-west of London.
Watch: Images projected onto Windsor Castle of Donald Trump and Jeffrey Epstein
The Trumps gave King Charles a replica of a President Eisenhower Sword in a gift exchange.
It symbolises "profound respect" and acts as a "reminder of the historical partnership that was critical" to winning the Second World War, according to Buckingham Palace.
"The sword also symbolises the enduring values and co-operative spirit that continues to define the relationship between the United States and Great Britain," the palace added.
King Charles and Queen Camilla gifted Mr Trump a bespoke, hand-bound leather book specially crafted by the Royal Bindery in Windsor Castle to celebrate the 250th anniversary of the Declaration of Independence, which was adopted in Philadelphia on 4 July 1776.
The president also received the union flag that was hoisted above Buckingham Palace on the day of his inauguration earlier this year.
Mrs Trump was given a silver and enamel bowl featuring the queen's cypher crafted by Northern Irish artist Cara Murphy, a renowned silversmith, as well as a personalised Anya Hindmarch handbag.
The Trumps were also presented with a silver photograph frame engraved with their joint cyphers.
Queen Camilla was gifted a Tiffany & Co vintage 18-carat gold, diamond and ruby flower brooch.
The brooch, said to represent diplomacy, friendship, and respect, features rubies and diamonds, the birthstones of the queen and Mrs Trump.
The couple are staying overnight in Windsor Castle as guests of the royals.
Watch: 'Great honour' to be hosted by 'friend', King Charles - Trump
The Trumps arrived at Stansted Airport last night, before making their way by helicopter, on Marine One, to central London.
They spent the night at the US Ambassador's residence in Regent's Park.
On arrival in London, President Trump spoke of his love for the UK, telling reporters that it was a "great honour" to be hosted by his "friend", King Charles.
Additional reporting Reuters, AFP
